West Nile outbreak in U.S. surpasses 1,100 cases

A Culex quinquefasciatus mosquito is shown on a human finger in this undated photograph from the Centers for Disease Control and Prevention. This type of mosquito is associated with transmission of the West Nile virus.

The United States is experiencing one of the biggest outbreaks of West Nile virus in history, according to the federal Centers for Disease Control and Prevention.

As of Tuesday, 1,118 cases of the mosquito-borne disease had been reported. That's the highest number ever reported at this point in the year since the disease was first detected in the U.S. in 1999. If cases continue to grow at this pace, the West Nile outbreak could be the largest ever in the United States, said Dr. Lyle Petersen, director of the CDC's Division of Vector-Borne Infectious Diseases.

So far, 41 people have died from West Nile infections. That compares to the 2011 season, for instance, in which 43 people died during the entire year.

About 630 people have developed the rare neuroinvasive form of the disease, which affects the nervous system. The other cases involved what doctors call West Nile fever, a condition that can cause fever, headache, body aches, nausea, vomiting.

Texas, the state hit hardest by the outbreak, has reported 537 cases of West Nile, close to 300 of which have been the neuroinvasive form. In some parts of the state, officials have begun aerial pesticide spraying to curb the spread of the disease.

Most people infected with the West Nile virus show no symptoms; from the number of neuroinvasive cases that have occurred, it can be estimated that more than 94,000 people in the United States have been infected with West Nile this year. It's likely that about 98 percent of cases of West Nile fever are not reported, the CDC says.

These unreported cases still take a toll on the nation's health and economy, as people who may not know that West Nile is the cause of their illness take time off from work or use health care resources, said William Schaffner, chairman of the department of preventive medicine at Vanderbilt University.

"The problem is even larger… than the already notably larger problem of neuroinvasive disease,"Schaffner said. The impact of unreported cases is a factor in the decision to call the current West Nile outbreak an emergency, as Dallas did, Schaffner said.